title: Protease-activated receptor 1 (PAR1) signalling desensitization is counteracted via PAR4 signalling in human platelets.
abstract: PARs (protease-activated receptors) 1 and 4 belong to the family of G-protein-coupled receptors which induce both G(α12/13) and G(αq) signalling. By applying the specific PAR1- and PAR4-activating hexapeptides, SFLLRN and AYPGKF respectively, we found that aggregation of isolated human platelets mediated via PAR1, but not via PAR4, is abolished upon homologous receptor activation in a concentration- and time-dependent fashion. This effect was not due to receptor internalization, but to a decrease in Ca²⁺ mobilization, PKC (protein kinase C) signalling and α-granule secretion, as well as to a complete lack of dense granule secretion. Interestingly, subthreshold PAR4 activation rapidly abrogated PAR1 signalling desensitization by differentially reconstituting these affected signalling events and functional responses, which was sufficient to re-establish aggregation. The lack of ADP release and P2Y₁₂ receptor-induced G(αi) signalling accounted for the loss of the aggregation response, as mimicking G(αi/z) signalling with 2-MeS-ADP (2-methylthioadenosine-5'-O-diphosphate) or epinephrine (adrenaline) could substitute for intermediate PAR4 activation. Finally, we found that the re-sensitization of PAR1 signalling-induced aggregation via PAR4 relied on PKC-mediated release of both ADP from dense granules and fibrinogen from α-granules. The present study elucidates further differences in human platelet PAR signalling regulation and provides evidence for a cross-talk in which PAR4 signalling counteracts mechanisms involved in PAR1 signalling down-regulation.
authors.affiliation: Department of Clinical and Experimental Medicine, Division of Clinical Chemistry, Medical Faculty, University of Linköping, University Hospital, Linköping 581 85, Sweden. knut.falker@liu.se
authors.forename: Knut
authors.initials: K
authors.lastname: Fälker
authors.forename: Linda
authors.initials: L
authors.lastname: Haglund
authors.forename: Peter
authors.initials: P
authors.lastname: Gunnarsson
authors.forename: Martina
authors.initials: M
authors.lastname: Nylander
authors.forename: Tomas L
authors.initials: TL
authors.lastname: Lindahl
authors.forename: Magnus
authors.initials: M
authors.lastname: Grenegård
mesh_terms: D001792:Blood Platelets / Q000201:enzymology*; D015536:Down-Regulation / Q000502:physiology; D006801:Humans; D020239:Receptor Cross-Talk / Q000502:physiology*; D044463:Receptor, PAR-1 / Q000502:physiology*; D018179:Receptors, Thrombin / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D010975:Platelet Aggregation Inhibitors
chemical_list: D044463:Receptor, PAR-1; D018179:Receptors, Thrombin; C112908:protease-activated receptor 4; D010975:Platelet Aggregation Inhibitors